For single-unit large module gears, such as those used in the vertical mill stands of the “Slabing-1150”, there is no statistical data on damage accumulation. Therefore, the information base for assessing the residual life of such gears should be the results of monitoring the control parameter of the technical condition, which has the most significant causal relationship with material damage. The technical condition monitoring method of single-unit large module gears by the parameter of the surface hardness of the teeth has been chosen. The theoretical basis of the monitoring process is a functional model that includes design, process, and operational factors and is formed on the basis of cyclically repeated procedural steps and measurement stages. The procedure for obtaining the results of hardness measurements of the surface layers of the teeth with maximum reliability has been proposed. It includes sets of goals, measurement data, supporting tools, and evaluation parameters. A practical check of the proposed monitoring model of achieving the maximum reliability of the results of the surface layers hardness measuring was carried out on the example of the intermediate gear tooth of the vertical mill stands “Slabing-1150” gearbox. The measurement error does not exceed 4.3%, which confirms the high efficiency of the proposed monitoring model.